The present invention relates generally to electrically programmable fuse (eFUSE) devices for integrated circuits and, more particularly, to a heat-shielded, phase change material (PCM) based reprogrammable fuse device with low power requirements.
The post-fabrication repair of logic and memory circuits using programmable fuses is an instrumental capability that currently supports acceptable yield in microchip technology. Future extensions of this repair capability toward full Built-in-Self Test (BIST) will likely require even more intensive development and use of eFUSEs, which in turn may result in the need for such devices to be reprogrammable (i.e., “multishot”), in contrast to the single shot fuses in current use. The more extensive use of eFuse technology in BIST may also result in the demand for a fuse having a lower footprint and a higher switching speed.
Fuse-based repair technology presently relies on several methods to make (“fuse”) or break (“antifuse”) electrical connections in fabricated structures. For example, laser-fusible links represent an early approach, which are now replaced by electrical techniques entirely internal to the chip. In addition, electromigration fuses (such as in IBM's eFUSE technology for rerouting chip logic), are currently in use. An electromigration fuse takes up a relatively large area and requires a high current to blow the fuse. Also, an electromigration fuse is “one-shot,” in that once the fuse is blown, it cannot be returned to a conducting state. Furthermore, the variation of eFUSE characteristics is relatively broad, thus requiring that the state of each fuse be sensed by a discriminating circuit with the digital result stored in a latch. The blowing of an electromigration fuse is also a relatively slow, on the order of about 200 μs, for example.
In contrast, an anti-fuse approach (e.g., used for some DRAM repair operations) typically involves a very thin dielectric material such as silicon dioxide, or a sandwich combination of silicon oxide-nitride-oxide (ONO), between two conductors. The anti-fuse is programmed by applying a relatively high voltage through the conducting terminals, causing dielectric breakdown in the dielectric, when the resistance of the anti-fuse permanently changes from high to low. This is also a one-shot technique requiring high voltage.
Unfortunately, the existing controllable link technologies described above may not have optimal properties for future microchip generations, due to factors such as: excessive area taken up by the fuse, “sunsetting” of the non-standard high voltages/currents which may be required by fuse programming, the desirability of “multishot” reprogrammable fuses, and insufficient speed for future BIST.
Reprogrammable fuses utilizing chalcogenide materials (and indirect heating through a resistive heater) are described in U.S. Pat. No. 6,448,576 to Davis et al. However, the use of such reprogrammable fuses as described in the '576 patent are essentially confined to locations in the lower logic layer of the device, on the semiconductor substrate, as only the materials (e.g., silicon, oxides, metals) used in the logic layer can tolerate the high temperatures (e.g., on the order of about 1000° C.) that result during chalcogenide switching.
However, the material corresponding to the location of existing eFUSEs in the top areas (back-of-the-line section) of the chip differs from logic level devices, in that low-K material is typically used as the insulating material between the conducting lines and vias. This type low-K material is heat sensitive (e.g., unable to support more than 400° C.), thus severely constraining the use of heat-driven devices where the maximum device temperature during programming can reach 1000° C., even for very short durations. Moreover, because such chalcogenide fuse materials in the '576 patent emit large amounts of heat, it is estimated that switching currents needed to produce the required heat are on the order of about 15 mA. Under this assumption, a required heater current of 15 mA would in turn result in a design that is inconveniently large, requiring a driver FET width on the order of about 15 microns.
Accordingly, it would therefore be desirable to provide a reprogrammable fuse (eFUSE) suitable for BIST applications, wherein the devices may be located in the back-end-of-line (BEOL) regions of a semiconductor device.